For engine systems in vehicular or other mobile applications where a supply of hydrogen is required, due to challenges related to on-board storage of a secondary fuel and the current absence of a hydrogen refueling infrastructure, hydrogen is preferably generated on-board using a fuel processor. The hydrogen-containing gas from the fuel processor can be used to regenerate (for example, including to desulfate and/or heat) one or more devices in an engine exhaust after-treatment assembly, can be used as a supplemental fuel for the engine, and/or can be used as a fuel for a secondary power source, for example, a fuel cell. In some applications, such as the latter, the hydrogen-containing gas stream from the fuel processor may require additional processing prior to use. In some applications the demand for the hydrogen-containing gas produced by the fuel processor is highly variable.
One type of fuel processor is a syngas generator (SGG) that can convert a fuel into a gas stream containing hydrogen (H2) and carbon monoxide (CO), known as syngas. Air and/or a portion of the engine exhaust stream can be used as an oxidant for the fuel conversion process. Steam and/or water can optionally be added. The SGG can be conveniently supplied with a fuel comprising the same fuel that is used to operate the engine. Alternatively a different fuel can be used, although this would generally involve a separate secondary fuel source and supply system specifically for the SGG.
The thermochemical conversion of a hydrocarbon fuel to syngas is performed in an SGG at high operating temperatures with or without the presence of a suitable catalyst. Typically, a high SGG operating temperature is desired in order to increase the fuel conversion efficiency of the process, which in turn can reduce the size of the SGG. However, excessive operating temperatures can cause undesirable effects including catalyst sintering, formation of loose amorphous soot and the requirement for the use of thermally robust specialty materials. Insufficient operating temperatures can cause undesirable effects including reduced chemical kinetics, reduced stability of the reaction flame, low fuel conversion, high concentrations of unconverted hydrocarbons in the product syngas, and formation of dense, more graphitic carbon or coke.
Parameters including equivalence ratio (ER), oxygen-to-carbon ratio (O/C ratio) and operating temperature are typically controlled and adjusted in an attempt to increase the efficiency of the fuel conversion process while reducing the generally undesirable formation of carbon and other deposits, which can cause undesirable effects within the SGG and/or in downstream components. Typically, the parameter ER is employed when an oxidant stream supplied to the SGG contains molecular oxygen, for example, an air stream, while the parameter O/C ratio is employed when an oxidant stream supplied to the SGG contains primarily chemically bound oxygen.
The term equivalence ratio (ER) herein refers to the ratio between the actual amount of oxygen supplied and the theoretical stoichiometric amount of oxygen which would be required for complete combustion of the fuel. An ER of greater than 1 represents a fuel lean mode (excess oxygen) and a resulting product stream comprising a flue gas stream, while an ER of less than 1 represents a fuel rich mode (excess fuel) and a resulting product stream comprising syngas. The term “product stream” refers to an output stream from a fuel processor or SGG. The term oxygen-to-carbon ratio (O/C) herein refers to the ratio between the total atomic oxygen and the total atomic carbon in the reactants supplied to the SGG.
Over time, carbon accumulation can impede the flow of gases, increase the pressure drop across the SGG and its associated components, and reduce the operating life or durability of the SGG. Large accumulations of carbon also have the potential to create excessive amounts of heat that can damage the SGG if the carbon is converted (for example, combusted or oxidized or gasified) in an uncontrolled manner, for example, in a short period of time. The term carbon herein includes solid fraction particulates of carbon including elemental carbon, coke and soot, as well as carbonaceous gums, resins and other deposits.
In some applications including, for example, regeneration of a lean NOx trap (LNT) and/or a diesel particulate filter (DPF) in an engine after-treatment assembly, the demand for syngas can be intermittent at varying and various intervals; can occur between prolonged intervals, for example, minutes or hours; and can last for only a short period of time, for example, seconds or minutes. During the production of syngas, heat is generated by the exothermic reactions of the fuel conversion process, which can raise and/or maintain the temperature of an SGG within a desired range. When syngas is no longer in demand, for example, upon completion of regeneration of an exhaust after-treatment device, the production of syngas can be ceased in order to reduce fuel consumption. However, ceasing the flow of one or more reactants to the fuel processor and the production of syngas can allow the temperature of the SGG to fall. If the temperature falls below a desired threshold, additional time may be required to re-establish the desired operating temperature of the SGG. Maintaining the temperature of the SGG above a threshold can advantageously reduce the response time to produce syngas when demanded.
In one known method used to maintain the temperature of the SGG, an SGG is operated so that syngas is intermittently produced or pulsed at regular intervals. A shortcoming of this method can be a higher relative fuel consumption. In another known method, an SGG is operated so that the air-fuel ratio of the reactants is adjusted to a fuel lean mode (with an equivalence ratio of greater than 1) when syngas is not demanded. Operation of the SGG in a fuel lean mode can produce a flue gas and heat that can sustain or increase the temperature of the SGG while reducing the consumption of fuel (as compared to producing syngas). However, if fuel is supplied to the SGG continuously, the fuel consumed to maintain the temperature of the SGG can represent a substantial portion of the total fuel consumption, for example, in some cases about 50%-70% of the total fuel consumed by the SGG.
In vehicular or other mobile applications, an on-board SGG should generally be low cost, compact, light-weight, reliable, durable, and efficiently packaged with other components of the engine system. Some particular challenges associated with the design and operation of fuel processors for vehicular or other mobile applications can include the following:                (a) Increasing the output of CO and/or H2 produced by the fuel processor, so that a smaller fuel processor can be used to satisfy a given syngas demand;        (b) Increasing the ratio of H2 to CO in the product syngas stream;        (c) Reducing the volume and weight of the fuel processor;        (d) Reducing the cost of the fuel processor;        (e) Producing H2 and CO intermittently, and responding rapidly to fluctuating demands for H2 and CO;        (f) Maintaining the temperature of a fuel processor above a desired threshold temperature, in order to respond rapidly to fluctuating demands for H2 and CO; and        (g) Reducing fuel consumption.        
The present methods of operating a fuel processor are effective in addressing at least some of the issues discussed above, both in engine system applications, as well as in other fuel processor applications.